The subject matter disclosed herein relates to tension members such as those used in elevator systems for suspension and/or driving of the elevator car and/or counterweight. More specifically the subject disclosure relates to systems for monitoring a tension member for wear and/or failure.
Elevator systems often include a car and a counterweight that are suspended in a hoistway via a load bearing member, such as a rope or belt. A drive machine moves the load bearing member to cause the selected movement of the car to different levels of a building. Conventional elevator systems use rope formed from steel wires as a lifting tension load bearing member. Other systems utilize a lifting belt formed from a number of steel cords, formed from steel wires, retained in an elastomeric jacket. The cords act as the load supporting tension member, while the elastomeric jacket holds the cords in a stable position relative to each other, and provides a frictional load path to provide traction for driving the belt.
During normal elevator operation, the load bearing members are subjected to a large number of bending cycles as they travel over drive sheaves and deflector sheaves of the elevator system. These bending cycles cause a degradation of the breaking strength of the ropes or cords within the coated steel belt via the mechanism of wire fretting or fatigue. Such fatigue is a major contributor to reduction in service life of the load bearing member. It is often desired to inspect the condition of the load bearing member for damage or degradation. This is done via visual inspection where practicable, but in many instances visual inspection is not possible or is insufficient, alternative methods are utilized.
Some electrical characteristics, such as electrical resistance or impedance of the rope or cords will vary with decreasing cross-section thereof. Accordingly, it is possible to determine the remaining support strength of the load bearing member based on the measured electrical characteristics. One system utilizing electrical characteristics of the rope or cords is called resistance-based inspection (RBI). An RBI system is secured to the rope or cords and monitors an electrical resistance of each cord in the belt. Since the electrical resistance of the rope or cord is proportional to its cross-sectional area, changes is electrical resistance can be correlated to reduction in cross-sectional area of the rope or cord, indicating an amount of fretting, and a corresponding remaining service life.
In a typical system, circuit leads are connected directly to the rope or cord. Direct connections, however, often result in point contact, or contacting, for example, only one or more strands of the rope while not contacting other strands at all. This results in inaccurate and non repeatable measurements.